This invention relates to an illuminating device for the optical, particularly image-analytical evaluation of a microbiological structure. The illuminating device comprises a diffused, radiating, surface light source and an aperture system positioned between the microbiological structure and the light source. Instruments for examining bacterial growth on suitable nutrient media are today an indispensable aid to microbiological research. The nutrient media are thereby injected either at predetermined positions or distributed over the entire surface. Characteristic patterns of growth then arise, (hereafter described as microbiological structure), which either have a periodical or a surface object structure. The character, that is the macroscopic appearance and the optical properties important for an image-analytical evaluation of the microbiological structure (absorption, refraction, diffusion) depend on which process is used for examining the bacterial growth. The following microbiological standard processes have become established:
1. agar dilution (AD) PA0 2. microdilution (MD) PA0 3. inhibition zone measurement on petri dishes (IZ-P) PA0 4. inhibition zone measurement on large rectangular plates (IZ-XY) PA0 5. colony counting (CC). PA0 (a) the aperture system is formed by two optically complimentary aperture screens disposed in series and with a spacing between them, PA0 (b) the first aperture screen consists of a large number of transparent, circular discs positioned at intervals on an opaque background, and the second aperture screen consists of a large number of opaque circular discs positioned at intervals on a transparent background, PA0 (c) and the opaque circular discs are larger in diameter than the transparent circular discs, so that the circular discs of the two aperture screens overlap in the projection. PA0 (a) the spacing h between the two aperture screens is selected such that it is from 0.5 to 5 times larger than the diameter of the transparent circular discs of the first aperture screen. PA0 (b) The difference d.sub.2 -d.sub.1 in diameter of the two circular discs of both aperture screens is selected within a range which is from 5 to 100% of the spacing of the two aperture screens.
These processes are described in detail in the technical literature and thus do not need to be described in more detail here. In common with all processes, the image-analytical problem, object positions or surface areas, in which bacterial growth arises, can be localized by scanning with a video camera and/or by quantitative evaluation. The extent to which bacterial growth arises depends on the effect of particular antibiotic quantities, which are incorporated into the nutrient media. When using the agar- and microdilution process, bacterial growth can only arise at determined positions. On the other hand, a variable surface bacterial growth takes place in the case of the inhibition zone measurement method. In the colony counting method, a pointwise bacterial growth occurs in the form of individual colonies with a diameter up to a few millimeters. The positions of the individual colonies are, however, distributed over the nutrient medium surface in an irregular manner.
The image-analytical evaluation of microbiological structures is frequently connected with problems, since the structures are often of very low-contrast and are embedded in an uneven ("irregular") environment, the brightness distribution of which substantially corresponds with the brightness distribution of the structure. in such cases, further improvements can hardly be achieved even by using electronic means. On the other hand, it has been shown that the contrast between objects and background substantially depends on the type of illumination. In the initial phase of the microbiological examination methods, optical transmission measurements were additionally carried out. Modern apparatus for examining biological samples generally operates by this illumination principle. After experimental indications had been obtained, such that the contrast ratios were allowed to increase, in all respects, by an improved illuminating device, further developments were carried out in this area and finally a new illuminating device was designed. Thus the problem was that the used stationary television camera scans all microbiological structures used from above by a light scattering system, evaluates them imagewise and the microbiological structures are illuminated from below by oblique incident light, such that direct linear incident light is avoided in the camera. The camera should thus only detect light which is scattered from within the microbiological structure, while the direct rays emitted from the light source are cut out. This principle should be realised on any substrate material for all growth patterns and plate sizes (square plates up to 30 cm.times.30 cm in size).